The invention relates to a circuit arrangement for controlling the frequency of an oscillator in a color television receiver or a video recording and/or reproducing apparatus, comprising a phase-comparator circuit having a first input for receiving a burst signal, a second input for receiving a first oscillator signal from a reference oscillator coupled to said second input, which phase-comparator circuit is constructed to produce a first output signal on an output depending on the phase difference between the color burst and the first oscillator signal, a correct circuit for generating an amplitude-limited correction signal, a signal-combination circuit for superimposing the correction signal on the first output signal and deriving therefrom a control signal for controlling the oscillator, the control signal being substantially independent of fluctuations as a result of the phase alternation of the burst signal every other line.
Such a circuit arrangement is known from DE-PS No. 32 02 210. Said Patent Specification describes a device for processing a PAL television signal, in which an oscillator signal generated by a reference oscillator and a chrominance signal modulated on a frequency of 4.43 MHz are applied to a phase comparator circuit. The chrominance signal contains the color signal occurring during the horizontal scan and the burst signal occurring during a part of the horizontal flyback. Phase comparison is effected in the time during which the burst signal appears. The output signal of the phase comparator circuit is applied to an adder whose output is connected to a low-pass filter. The low-pass filter supplies a control signal to a frequency-dependent oscillator.
Since in PAL systems the phase of the burst signal changes 90.degree. from line to line, the output signal of the phase comparator circuit also changes at half the line frequency. In order to ensure that this alternating signal does not influence the frequency-dependent oscillator and thereby cause colour changes, the known circuit arrangement comprises a correction circuit which generates a correction signal. This correction signal is added to the output signal of the phase comparator circuit and largely eliminates the fault caused by the alternating signal. The correction circuit comprises a circuit which generates pulses at half the line frequency. These pulses are applied to a current source via a phase inverter to generate a direct current which is superimposed on the output signal of the phase comparator circuit by the adder.
In order to ensure that a signal opposite to the alternating component in the output signal of the phase comparator circuit is applied the output of the low-pass filter in the correction circuit is coupled to a pulse counter via a high-pass filter, a half-wave rectifier and a pulse shaper. This counter counts the pulses in the output signal of the low-pass filter. After the pulse counter has counted four pulses it changes over a multivibrator circuit by means of which the state of the phase inverter is switched over. The pulses of half the line frequency applied to the current source are inverted.
The known circuit arrangement is very intricate and responds slowly in the case of incorrect switching phases, which occur for example during reproduction by means of a video recording and/or reproducing apparatus (for example, monitoring during fast forward operation). The phase of the burst signal is then not shifted by 90.degree. every other line. In the case of such an incorrect switching phase at least 8 line intervals will elapse before the known circuit arrangement is capable of correcting this fault and thus eliminating color faults.